Under Shopify admin in the Embedded app section, click Manage embedded app.
If you'd like to load your app inside Shopify admin, then the setting must be enabled.

Caution

The Embed in Shopify admin app extension should only be enabled for apps that use Shopify App Bridge. If your app doesn't use Shopify App Bridge, then it won't load properly in Shopify admin. Once enabled, Shopify loads your app inside an iframe.